Electric Fields and Forces
An electric field describes the region of space extending from a charged particle in which another charged particle experiences a force related to its charge. Electric fields surround all charged particles.
Surrounding every charged particle there is a region of space in which there is a potential electrostatic force. The strength of the electric field surrounding a point charge, as measured by the electrostatic force, is directly proportional to the charge and inversely proportional to the square of the distance from the charge. The electrostatic force requires no medium to act (i.e., it can act in a vacuum).
Just as all particles with mass exert a gravitational influence on all other particles, all charged particles exert an electrostatic force on all other charged particles. Because the strength of the force is the inverse of the square of the distance between the charged particles, however, there are some practical limitations regarding the measurability of the an electrical field or force. In practical terms an electrical or electrostatic force arises whenever a charged object is sufficiently near another charged particle so that the resulting force is measurable. The strength of the electric field, E, at the position of a charged object in that field is defined to be the magnitude of the electric force F acting on the object divided by the magnitude of the charge Q of that object, E = F/|Q|.
The electric field is a vector quantity that is expressed in terms of force per charge (usually Newtons per Coulomb. The direction of this vector at any point is defined to be the direction of the electric force that would be exerted on a positive charge placed at that point. Accordingly, if the origin of the electric field is a negatively charged particle the force on a positively charged particle is directed toward the negative charge (opposite charges attract). If the origin of the electric field is a positive charge the force on another positive charge is directed away from the field originating charge (like charges repel).
Coulomb's law states that the electric force between two stationary charged particles q and q separated by a distance r is given by the equation F=k|q||q|/r2 , where k (9x109 N m2 C-2 ) is the Coulomb constant.
Electric forces can be added vectorially. To determine the electric field at a point P when there are two charged particles in the vicinity of this point, the electric fields at P due to each charged particle must be determined separately. The respective vector components of each field can be vectorially added to give the overall field strength at point P.
When a battery or some other device possessing a electromotive force is connected to two conducting plates held parallel to one another, an electric field is set up. The magnitude of the electric field in this case is given by the potential difference divided by the plate separation. A positive charge will gain electrical potential energy when it is moved in a direction opposite to the electric field. A negative charge will, however, lose electrical potential energy when it is moved in the direction opposite to the electric field. To move a particle from one place in an electric field to another place in the electric field takes work. The energy required to do this work is the electric potential between these two points.
The physical description of electric fields and forces can be traced back to 300 B.C. when Greek philosopher Theophrastus asserted that amber acquired a power to attract light objects when rubbed. The first published study of electrical phenomena appeared in 1600 A.D. when English physician and scientist William Gilbert first published his studies on magnetism and electrical attractions. Gilbert was the first to use the term electric, which he derived from the Greek elektron meaning amber, when describing a force that such substances exert after being rubbed. Gilbert differentiated between magnetic and electric action as well.
In 1672 German physicist Otto von Guericke (1602-1686) described the first machine for producing an electric charge. In the eighteenth century the empirical foundations for intensive studies in electromagnetic phenomena were kindled by the work of American scientist Benjamin Franklin. Franklin spent much of his life researching electrical phenomena and developed a theory that all matter consisted of a fluid that was believed to be electricity. He explained the forces associated with electricity to be either an excess or lack of this fluid. In 1766 English scientist Joseph Priestley (1733-1804) developed a law relating the force between electric charges and the distance between them. He also discovered that no electric field of force exists within a metal sphere whose surface is charged. In the 1780s, French physicist Charles-Augustin de Coulomb invented the torsion balance and used it to accurately measure the force exerted by electrical charges.
In the nineteenth century there were consistent advancements in the understanding of electric fields and forces. In 1800, Italian physicist Alessandro Volta demonstrated the first electric battery. The work of German mathematician Carl Friedrich Gauss, French scientist André-Marie Ampére, Danish scientist Hans Christian Ørsted and English physicist Michael Faraday resulted in the unification of electrical theory with magnetic theory in Scottish physicist James Clerk Maxwell's set of equations describing electromagnetism.
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